The present invention relates to a novel method of producing methyl methacrylate from acetone and methanol as the starting materials. Methyl methacrylate is one of the industrially most important intermediate materials and has been used in a great amount as a starting material for various types of polymers.
Known methods currently employed in industrially producing methyl methacrylate include ACH (acetone cyanhydrin) method in which methyl methacrylate is synthesized through ACH produced from hydrogen cyanide (prussic acid) and acetone; a modified ACH method; C4 oxidation method using isobutylene or t-butyl alcohol as the starting material; etc. In addition, an oxidative dehydrogenation of isobutyric acid, a dehydration by condensing formaldehyde with propionic acid or propionaldehyde, an ammoxydation of isobutylene, etc. have been proposed.
The C4 oxidation method involves many side reactions to detrimentally decrease the yield of methyl methacrylate and increase a cost for purification, and requires a complicated, expensive production apparatus. Additional disadvantage of the C4 oxidation method is that the supply of the starting materials such as isobutylene and t-butyl alcohol is subject to limitation. An oxyesterification in which the oxidation and esterification of methacrolein occur simultaneously has been proposed as a modified method for the C4 oxidation method. However, the basic problems mentioned above still remain unsolved. Also, a method including an addition reaction of carbon monoxide and methanol to methylacetylene has been proposed. However, it is difficult to supply the starting material in an amount enough to practice the method in industrial scale.
In the ACH method, methyl methacrylate is produced by reacting, in the presence of an excess amount of concentrated sulfuric acid, methanol and ACH synthesized from hydrogen cyanide and acetone. Since the respective reactions can be easily controlled and the yield of methyl methacrylate is relatively high, the ACH method is still widely used. However, this method accompanies a large amount of waste sulfuric acid and by-produced ammonium sulfate which increase a cost of producing methyl methacrylate. To eliminate this drawback, Japanese Patent Laid-Open No. 1-290653 proposes a modified ACH method in which the use of sulfuric acid catalyst is avoided, i.e., no ammonium sulfate is by produced. The proposed, modified ACH method comprises the following five steps:
Step (1) of producing acetone cyanhydrin from hydrogen cyanide and acetone according to the following reaction, EQU HCN+CH.sub.3 COCH.sub.3 .fwdarw.(CH.sub.3).sub.2 C(OH)CN;
Step (2) of producing .alpha.-hydroxyisobutyramide by hydrating acetone cyanhydrin according to the following reaction, EQU (CH.sub.3).sub.2 C(OH)CN+H.sub.2 O.fwdarw.(CH.sub.3).sub.2 C(OH)CONH.sub.2 ;
Step (3) of producing methyl .alpha.-hydroxyisobutyrate and formamide by a reaction of .alpha.-hydroxyisobutyramide and methyl formate according to the following reaction, EQU (CH.sub.3).sub.2 C(OH)CONH.sub.2 +HCOOCH.sub.3 .fwdarw.(CH.sub.3).sub.2 C(OH)COOCH.sub.3 +HCONH.sub.2 ;
Step (4) of producing methyl methacrylate by dehydrating methyl .alpha.-hydroxyisobutyrate according to the following reaction, EQU (CH.sub.3).sub.2 C(OH)COOCH.sub.3 .fwdarw.CH.sub.2 .dbd.C(CH.sub.3)COOCH.sub.3 +H.sub.2 O;
and
Step (5) of producing hydrogen cyanide by dehydrating formamide according to the following reaction, EQU HCONH.sub.2 .fwdarw.HCN+H.sub.2 O.
The proposed method is excellent because the yield of each step is relatively high, the reaction apparatus scarcely needs to be corrosion-resistant, and hydrogen cyanide is reproduced. However, the conversion rate of the step (3) is as low as 50% because it is an equilibrium reaction, thereby to necessitate additional steps for separating and recycling the non-reacted .alpha.-hydroxyisobutyramide and methyl formate. Therefore, a large-sized apparatus is required and a utility cost is increased. In addition, since the dehydration of formamide in the step (5) should be carried out under reduced pressure, a vacuum apparatus is required to increase an energy cost.